Broken Hasselblad A12 back question

I finally bought two A12 backs for my Hasselblad 500cm and I now have a whole kit. One of the backs is a newer one with an ER serial number but the other one is an older 80's model that I bought for real cheap and it has 3 holes on the back of it which is, of course, letting light in. It's missing the plastic ASA dial, I guess, and it looks like there were once three rivets that held it in place.

Anyways, anyone have any ideas on how I can plug up the holes? In the short-term I was going to tape it up but I am unsure what kind of tape will completely seal out all of the light.

I have an early "peep hole" 12 back where the entire hinged asa reminder that covers the peep hole is missing.
I made a piece if cardboard that fits and keep it in place with gaffers tape. No leaks in 7-8 years.

Your issue sounds like the holes are way smaller (although I can't see them in my mind this early ) than in my case so I'd image tape will work fine.